In a wide variety of optical systems, it is desirable to be able to detect sets of optical interference fringes. One such system, the interferometer, is commonly used to test optical systems for proper alignment of optical components and imperfections in optical components.
A conventional interferometer test apparatus is shown in FIG. 1. An interferometer I generates a coherent beam of light C which is projected along an optical path through an optical system S to be tested. Optical system S includes various optical elements, generally denoted E. Interferometer I and optical system S are aligned along an azimuthal axis Z.
Initially, collimated beam C passes through a focal plane F located in close proximity to a refractive optical element E.sub.1. Coherent beam C then enters optical system S through optical element E.sub.1, and is subsequently refracted through, or reflected from, the remaining elements as it follows the optical path of the system.
A reflector R is positioned on an opposing end of the optical system to reflect coherent beam C back through the optical system. The reflected coherent beam follows the same optical path, but in a reverse direction, and ultimately reenters interferometer I.
Interferometer I combines the returned coherent beam C with a reference beam (not shown) to generate an interferometric image having a plurality of interference fringes. The configuration of the interference fringes provides information as to the alignment and quality of the optical system S. The interferometric image is detected and displayed on a television monitor M.
In an interferometer test apparatus such as the one shown in FIG. 1, it is critical that interference fringes, other than those caused by imperfections in the system under test, be substantially eliminated. Unfortunately, due to the proximity of optical element E.sub.1 to focal plane F, unwanted reflections from element E.sub.1 combine with the returned portion of the coherent beam to obscure desired fringes in the resulting interferometric image. This effect is shown most clearly with reference to FIG. 2, which provides a closeup view of the prior art system of FIG. 1, showing the focal plane F, a portion of optical element E.sub.1, and various components of exemplary optical rays A and B.
Optical ray A passes through focal plane F and into optical element E.sub.1. At the leading surface of optical element E.sub.1, optical ray A is split into two components, a transmitted ray A.sub.T and a reflected ray A.sub.R. Transmitted ray A.sub.T continues along the optical path of the system and is eventually reflected by reflecting surface R for return along the optical path (FIG. 1).
A return portion A'.sub.T of transmitted ray A.sub.T is also shown in FIG. 2. For clarity, return ray A'.sub.T is shown parallel to, but offset from, outgoing ray A.sub.T. However, it should be understood that return transmitted ray A'.sub.T returns along substantially the identical path as outgoing transmitted ray A.sub.T.
Return ray A'.sub.T passes through element E.sub.1 and continues, as shown, for return to interferometer I. Optical ray B is likewise split into a transmitted beam B.sub.T and a reflected beam B.sub.R. Transmitted beam B.sub.T passes through the optical system and then returns as beam B'.sub.T. Beam B'.sub.T returns along the same optical path and again passes through optical element E.sub.1 and continues on to interferometer I.
As a result of the angle of reflection from optical element E.sub.1, reflected ray A.sub.R emerges parallel with return transmitted ray B'.sub.T. Likewise reflected ray B.sub.R emerges parallel with return transmitted ray A'.sub.T. Thus the reflected portion of coherent beam C emerges 180 degrees opposite from the return transmitted portion of coherent beam C. In other words, the coherent beam is rotationally sheared. Consequently, the reflected portion of rays A and B interfere with the return transmitted portions of beams B and A, respectively, to produce interference fringes which obscure the fringes of interest.
Reflections also occur at the trailing edge of element E.sub.1 and from the leading and trailing surfaces of all refractive elements along the optical path. These other reflections may also produce interference fringes which obscure desired fringes. However, for clarity, only reflections from the leading edge of optical element E.sub.1 are shown in FIG. 2.
Further, although this interference effect is shown by way of an example having a flat optical element, undesired reflections from curved refractive optical elements also obscure desired interference fringes, especially curved elements which have large radii of curvature.
Since the optical element E.sub.1 is offset slightly from focal plane F, the reflected portion of light rays A and B emerge slightly radially offset from the transmitted portions of light rays A and B. If the optical element E.sub.1 is sufficiently far from focal plane F, then the reflected portion of light rays A and B may be sufficiently offset from the transmitted portions of rays A and B such that the unwanted rays do not enter interferometer I and, hence, do not cause distortion to the interferometric test.
Whether reflections from optical elements which are remote from the focal plane ultimately enter the interferometer to obscure desired fringes depends on the geometry of the optical system.
Typically, coherent beam C must be focused at the optical focus of the system to be tested. For many optical systems, the focus is at a point close to at least one refractive element of the system and the thus-described problem therefore occurs. Accordingly, it is highly advantageous to provide a means for eliminating the problem.
Although described with respect to interferometers, the problem of unwanted reflections from refractive optical elements affecting an optical image occurs in numerous applications and with numerous optical systems. Generally, the problem may occur in any optical system where a light beam is twice passed through a refractive optical element.